1580s, "a middle ground, quality, or degree," from Latin medium "the middle, midst, center; interval," noun use of neuter of adjective medius (see medial (adj.)). Meaning "intermediate agency, channel of communication" is from c.1600. That of "person who conveys spiritual messages" first recorded 1853, from notion of "substance through which something is conveyed." Artistic sense (oil, watercolors, etc.) is from 1854. Happy medium is the "golden mean," Horace's aurea mediocritas.

adj.

1660s, "average," from medium (n.). The Latin adjective was medius. Meaning "intermediate" is from 1796. As a size designation from 1711. as a designation of cooked meat, it is attested from 1931, short for medium-rare (1881).

A substance, such as agar, in which bacteria or other microorganisms are grown for scientific purposes.

A substance that makes possible the transfer of energy from one location to another, especially through waves. For example, matter of sufficient density can be a medium for sound waves, which transfer mechanical energy. See more at wave.